The present invention relates generally to electric power supply systems, and more specifically to a programmable slewing power supply to drive microwave phase shifters.
Phased array radar and communications systems steer emitted electromagnetic radio frequency (RF) signals electronically by shifting the phase of these signals with respect to a matrix of transmitter elements housed in the array. Some of these systems use microwave phase shifters to adjust the phase of signals received from a high-power signal source. However, just as the phase of these signals needs to be adjusted to produce different waveforms, the voltage levels also need to be adjustable.
The task of providing a programmable high-efficiency power supply for microwave phase shifters is alleviated, to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference: U.S. Pat. No. 3,237,088 issued to Karp; U.S. Pat. No. 3,448,372 issued to Goff; and U.S. Pat. No. 4,513,360 issued to Ikenoue.
The above-cited references all disclose power supply systems. Typical prior art systems also include supply systems which provide supply voltages using a fixed power supply with a series linear amplifier. Such supply voltages are variable but the supply systems are characterized as being low in efficiency, excessive in terms of power dissipation, and as having poor reliability in the linear amplifier due to the wide variation required for output currents and voltages. The reasons for such performance problems are discussed below.
Magnetic coils used in phase shifters present large inductive loads with a series resistance. Linear drive is typically implemented using a fixed voltage power supply and a transconductance amplifier. Current is commanded as a voltage to be developed across the sense resistor. The voltage provided by the power supplies can be divided into three portions. First, part of the available voltage is used to drive the load as required (V=I*R; V=L*dI/dt), with the load power being the product of this voltage and the drive current.
A second portion of the voltage is required by the linear amplifier to stay in its active region of operation, free of saturation effects. The third portion, consisting of the voltage provided in excess of the preceding two portions, must also be used by the linear amplifier. This voltage serves no useful purpose, being dissipated as waste heat in the amplifier, but must be available to accommodate changing load requirements.
The third portion of the voltage is dissipated by the linear amplifier as follows. For a resistive load, it can be shown that the maximum power dissipated by the linear amplifier with a fixed voltage power supply is at one half maximum current, Pdiss=1/2Imax * V. For an inductive load the current voltage waveforms are out of phase, so that the maximum amplifier dissipation is Pdiss=Imax * V.
The peak voltage requirements can be much higher than the average voltage resulting in very high power levels. For a particular phase shifter drive, the maximum current of 7 amperes and peak linear voltage of 500 volts result in an amplifier dissipation of in excess of three kilowatts, which must be sustainable for an indefinite period.
Clearly, it would be of great benefit to have power supplies which provide only the first two portions of voltage discussed, without the third. In general, this can be accomplished with any D.C.-D.C. converter which uses inductive filtering and is capable of providing efficient power conversion over a range of input and output voltages.
Two important concerns arise, both related to the rate at which the output must change. The first is the bandwidth of the power supply regulation loop. For stability in switchmode power supplies, the unity gain crossover is limited to a theoretical maximum of one half of the switching frequency. In practice, the limit is lower because of the need to allow for variances.
The second concern is the slew rate of the power supply under load. This is a function of the peak power output required and the rate of change of voltage needed. The supply must provide current at the proper voltage, while also providing current to charge the output capacitance to slew the voltage. This requires input line filtering to isolate surges from the prime power line.
From the foregoing discussion it is apparent that there currently exists the need for a high frequency switchmode power supply with high peak power capabilities and wide output range to support systems including those with microwave phase shifters. The present invention is intended to satisfy that need.